So one more time, explain like I'm five, what does next.js actually Do?
Next level (no pun intended): Makes working with React easy.
And another: Next.js provides tools for web developers (rendering UI components, fetching data from APIs, optimizing and displaying images and fonts, etc.) to create fast web sites/apps.
I'm not dismissing Next.js or you, I just feel like these aren't great points in a vacuum.
Obviously these things can be done without using Next, React or even Javascript, like you say, but it all comes in a really well-integrated solution that mostly just works.
The point is that it's not clear what exactly Next does, let alone why it does those things. In my mind, Next (and friends) just helps smooth the edges of all aspects of modern web development. That's just not a very specific statement. Also, a lot of those edges aren't so sharp for a lot of use cases since modern browsers and servers offer the same functionality. I still think Next and friends are useful because the edges may not be as sharp, but there are quite a lot of them.